1. Field of the Invention
The present invention relates to electrically operated spool valves for controlling flow of a fluid, and more particularly to such electrohydraulic valves that operate a mechanism which alters timing of an internal combustion engine by varying the phase relationship between a cam shaft and a crankshaft.
2. Description of the Related Art
Internal combustion engines used in motor vehicles have a plurality of cylinders containing pistons that are connected to a crankshaft. Each cylinder has two or more valves that control the flow of a fuel mixture into the cylinder and the flow of post-combustion gases therefrom. Traditionally the cylinder valves were controlled by a camshaft which was mechanically coupled to rotate with the crankshaft. Gears, chains or belts have been used to couple the crankshaft to the camshaft so that the two rotate in unison. It is important that the valves open and close at the proper times during the combustion cycle within each cylinder. Heretofore, that timing relationship was fixed by the mechanical coupling between the crankshaft and the camshaft.
The setting of the camshaft timing often was a compromise that produced the best overall operation at all engine operating speeds. However, it has been recognized that more optimum engine performance could be obtained if the valve timing varies as a function of engine speed, engine load and other factors. With the advent of computerized engine control, it became possible to determine the optimum engine valve timing based on the operating conditions occurring at any given point and time.
With reference to FIG. 1, the engine computer 11 determines the optimum valve timing and issues a signal to an electrohydraulic valve 10 which controls the flow of pressurized engine oil from a pump to a cam phase adjustment mechanism 12. The cam phase adjustment mechanism 12 couples the camshaft 14 to a pulley 16, gear or other device that is driven by the engine crankshaft. The phase relationship between the rotating pulley 16 and the camshaft 14 can be dynamically varied by selectively applying pressurized engine oil to one of two ports 18 or 19 of the adjustment mechanism. For example, application of engine oil from the pump to the first port 18 and exhausting engine oil from the second port 19 to the tank advances the valve timing. Whereas connecting the second port 19 of the adjustment mechanism 12 to the pump and coupling the first port 18 to the tank retards the valve timing. The hydraulic valve 10 is a proportional type valve which allows the amount that the cylinder valves are advanced or retarded to be varied proportionally by metering the flow of engine oil to and from the adjustment mechanism 12. A sensor 15 provides an electrical signal indicating the angular phase of the camshaft.
Key to the operation of the variable cylinder valve timing is the proper control of engine oil flow to the two port 18 and 19 and the accurate metering of that flow. Thus the control valve 10 becomes a critical element in the proper operation of the engine.